Balaenoptera borealis
sei whale

These whales are found in all oceans and adjoining seas, except polar and tropical regions. These animals occupy temperate and subpolar regions in the summer, but migrate to sub-tropical waters during the winter.

These pelagic whales are found far from shore.

The largest known Sei whale measured 20 meters in length, although most whales are between 12.2 and 15.2 meters long. Of this length, the head and body make up about 13 meters. Males are slightly smaller than females. Sei whales have a relatively slender body with a compressed tail stock that abruptly joins the flukes. The snout is pointed, and the pectoral fins are short. The dorsal fin is sickle shaped and ranges in height from 25 to 61 centimeters.

The body is typically a dark steel gray with irregular white markings ventrally. The ventrum has 38-56 deeps grooves, which may have some feeding function. Each side of the upper part of the mouth contains 300 - 380 ashy-black baleen plates. The fine inner bristles of these plates are whitish.

Little is known about the actual social system of these animals. Groups of two to five individuals are typically observed, but sometimes thousands may gather if food is abundant. However, these large aggregations may not be dependent on food supply alone, as they often occur during times of migration. Norwegian workers call the times of great Sei whale abundance "invasion years."

Sei whales are among the fastest cetaceans, swimming at speeds of up to 50 kilometers per hour. Although distinguished by their speed, Sei whales are not remarkable divers. These whales dive only to shallow depths, and they remain submerged only five to ten minutes at a time.

The Sei whale obtains food by skimming through the water and catching prey in its baleen plates. These whales feed near the surface of the ocean, swimming on their sides through swarms of prey. An average Sei whale eats about 900 kilograms of copepods, amphipods, euphausiids and small fish every day.

The current economic importance of this whale is questionable. However, in the past, these large whales provided a great deal of income to the whaling industry. It cannot be stressed enough, however, that the positive economic effects of hunting this animal have been acheived only by large scale decimation of Sei whale populations. By overharvesting the whales, the whaling industry experienced a short term economic gain at a long term cost-- the reduction in the number of whales available for harvest.

Sei whales are listed as CITES appendix 1 from the equator to Antarctica. All other populations are listed as CITES appendix 2. The global population of these whales is estimated at only 57,000. Hunting of these whales by humans has been high since the 1950s. The take of these animals peaked in the 1964-65 season, when 25,454 of these whales were taken. The reported global catch of Sei whales in the 1978-79 season was only 150, showing the dramatic drop in whale populations. Some researchers have concluded that Sei whale populations are rising as a result of decreases in Blue and Fin whale poulations. However, this conclusion must be taken with caution, as actual data are scarce, and the dietary overlap between Sei whales and these other species is not complete.